"Of all forms of caution, caution in love is perhaps the most fatal to true happiness" — Bertrand Russell, lived in early 1800s.

It's out of character for someone like Bertrand Russell, a lover of mathematics, science, and logic, to dabble in something so negotiable as happiness.

But his idea that happiness can be found in the surrender to visceral feelings of love rings true — and contemporary science seems to be on his side.

"Happiness is the feeling that power increases — that resistance is being overcome" — Friedrich Nietzsche, alive in late-19th century.

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For Nietzsche, the famous mustachioed nihilist, happiness is a kind of control one has over their surroundings.

The German philosopher wrote frequently on the impacts that power (and a lack of power) can have on people's lived experiences. When people resist, they take back their agency. That sense of self can then turn into happiness.

"The secret of happiness, you see, is not found in seeking more, but in developing the capacity to enjoy less" — Socrates, lived in 450 BC.

For Socrates, one of the greatest ancient thinkers, happiness doesn't come from external rewards or accolades. It comes from the private, internal success people bestow upon themselves.

By paring down our needs, we can learn to appreciate simpler pleasures.

"The man who makes everything that leads to happiness depends upon himself, and not upon other men, has adopted the very best plan for living happily" — Plato, lived in 4th century BC.

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It's no surprise that Plato, Socrates' student, defines happiness like his teacher.

Plato's version of happiness is a form of personal growth. It's deriving satisfaction from achievements — running a faster mile, reading more books than last year — not for what those accomplishments can earn you.

By the time the message got to Aristotle, Plato's student, the idea of happiness had solidified into something we nurture on our own.

It's not a gift other people or things give to us, in other words. It's something we create from within and have a responsibility to protect.

"I have learned to seek my happiness by limiting my desires, rather than in attempting to satisfy them" — John Stuart Mill, born in 1806.

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John Stuart Mill was a titan of liberalism, perhaps its most important figure in history. He spread the gospel of liberty wherever he could.

When it came to happiness, he adopted the wisdom of the ancient Greeks. Instead of inundating his life with goods, Mill believed in utilitarianism. He believed in using things for a purpose, and if they served no purpose, he banished them from his life.

"The more man meditates upon good thoughts, the better will be his world and the world at large" — Confucius, lived in China around 500 BC.

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Confucius' sentiment about happiness has echoed through history in books such as "The Power of Positive Thinking" and recent research in cognitive behavioral therapy, which seeks to help people see the connections between their thoughts, feelings, and behaviors.

According to a Confucian mindset, happiness is a self-fulfilling prophecy that replicates itself the more we find reasons for its existence.

"The greatest blessings of mankind are within us and within our reach. A wise man is content with his lot, whatever it may be, without wishing for what he has not" — Seneca, born in Hispania in 4 BC.

Some research suggests people report the greatest happiness when they are engaged in things that require their full and present attention: good conversation, creative tasks, or sex.

"Life is not a problem to be solved but a reality to be experienced" — Soren Kierkegaard, early 19th-century philosopher.

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Before there was the stellar mash-up Twitter account Kim Kierkegaardashian, there was the Danish philosopher who inspired it.

Kierkegaard was of the mind that happiness comes from being present in the moment and enjoying the ride. Once we stop turning our circumstances into problems and start thinking of them as experiences, we can derive satisfaction from them.

"Happiness is like a butterfly; the more you chase it, the more it will elude you, but if you turn your attention to other things, it will come and sit softly on your shoulder" — Henry David Thoreau, born in 1817 in Massachusetts.

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Perhaps unsurprisingly, the transcendentalist author and advocate for civil disobedience took a passive approach to happiness.

As he detailed in "Walden," Thoreau preferred to break convention. He avoided habit. Perhaps through the commitment to randomness, he thought, he could find some larger, more cosmic sense of happiness.

The idea aligns nicely with other thinkers' message to live in the moment.